The present invention relates to a method of forming a shaped configuration at an end of a long element by cold forging.
As a long element having a shaped configuration at an end thereof, there is, for instance, the armature shaft of a starter motor for starting the engine of an automobile. Generally used for this armature shaft is a long one having at one end thereof a gear or serrations for driving which are formed coaxially and integrally with the shaft. Such a shaft is preferable to be manufactured by cold forging in view of high accuracy in size and excellence at strength.
FIG. 4 shows an example of the shaft which is generally designated by reference numeral 1. The shaft 1 includes a shaft body 2 and is formed at its end with a gear 3. Hitherto, when manufacturing this shaft 1 by cold forging, a portion 5 of a large diameter is beforehand formed at one end of a material 4 as shown in FIG. 5. Then, as shown in FIG. 6, the gear 3 is cold-forged in the end of the material 4 with a load applied on a stepped face between the large diameter portion 5 and a portion 6 of a small diameter. Alternatively, thought of is a method of placing a metal material in a die which has an inner space corresponding to the shape of the shaft 1 and forcing the material under high pressure by a punch so that the material fills the inner space of the die to be forged into the desired shape. Then, the material thus shaped is pushed out of the die by means of a knock-out pin which is inserted into the die in opposition to the punch.
In the former of the above described methods, however, an extra process is necessary to form the large diameter portion 5. Further, this large diameter portion remains in the completed shaft.
In the case of the latter method, a large frictional resistance produces between the material and the inner surface of the die according as the material extends within the die and, therefore, the forging load inevitably becomes large for surpassing the resistance to force the punch. This brings about a possibility that such defects in forging as scores and the like are caused in the material or the punch breaks down.
In order to reduce the resistance, therefore, a measure of applying a lubricant onto the periphery of the material has been taken when forcing the punch. But, the application of the lubricant gives rise to a disadvantage that the lubricant comes to exist between the material and the die inner surface to deteriorate the accuracy in size.
Moreover, the moving stroke of the knock-out pin for pushing the forged product out of the die has to be equal to, even at the minimum, the length which is necessary for the shaft body 2 to get out of the die, namely the length of the shaft body 2 itself.
Additionally, when pushing the product by the knock-out pin, the whole peripheral surface of the shaft body 2 comes into slide contact with the inner surface of the die to produce a large frictional resistance and, therefore, the load for forcing the knock-out pin also has to be made large, involving the enlargement in size of an apparatus for this end. Thus, the method can not be efficient. Further, the knock-out pin must conform in shape to the teeth of the gear or be smaller in diameter than the root circle of the gear. But, it is costly to form teeth in the knock-out pin all over the length thereof corresponding to its moving stroke in conformity with the teeth of the gear. On the other hand, if forming the knock-out pin of a small diameter, there is a fear that the pressure acting on the pin will increase and the pin will break down.